Method for producing melamine

ABSTRACT

A process for the production of melamine by pyrolysis of urea in a high-pressure reactor having a vertical central pipe is provided. The melamine flows upwards into the reactor from below, mixes in the lower part of the reactor with a urea melt, and optionally NH 3 , introduced into the reactor from below, and emerges from the central pipe in the upper part of the central pipe. Part of the melamine formed flows downward in the annular space between the central pipe and reactor wall, and the remainder is expelled for further work-up. The off-gases are removed at the top of the reactor. A reactor for carrying out the process is also provided.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a National Phase Patent Application ofInternational Application Number PCT/EP01/11890, filed on Oct. 15, 2001,which claims priority of Austrian Patent Application Number A 1802/2000,filed Oct. 20, 2000.

FIELD OF THE INVENTION

[0002] The invention relates to a process and a device for theproduction of melamine by pyrolysis of urea.

BACKGROUND OF THE INVENTION

[0003] In the high-pressure processes for the production of melamine,urea is reacted to give melamine by means of an endothermic liquid-phasereaction. The liquid melamine, depending on the pressure and temperatureconditions in the reactor, additionally contains different amounts ofdissolved NH₃ and CO₂, and condensation by-products and unreacted urea.The melamine thus obtained is then solidified, for example, by quenchingwith water or with ammonia, by sublimation with subsequent desublimationor by releasing the pressure under specific conditions.

[0004] The reactor used is customarily a tank reactor with a centralpipe and heating elements arranged outside the central pipe, whichprovide the heat necessary for the reaction. These heating elements arepipe bundles, in which a salt melt circulates, arranged parallel to thecentral pipe. Urea and NH₃ are introduced at the bottom of the reactor,impinge on a distributor plate which is located underneath the centralpipe and react in the free space between the pipe bundles, in whichmelamine is already situated, with decomposition and evolution of gas togive melamine. In WO 99/00374, such a reactor is depicted schematically,the flow direction of the melt also being indicated such that thereaction mixture outside the central pipe flows upwards between the pipebundles and separates there into off-gas and liquid melamine. Theoff-gas is removed at the top of the reactor, one part of the melaminemelt is removed from the reactor via an overflow and the other part ofthe melamine melt flows downwards within the central pipe on account ofgravity.

[0005] This previously used type of reactor, however, has thedisadvantage that the pipe bundles, in particular in the case ofrelatively high urea throughputs, corrode relatively rapidly andtherefore have to be frequently exchanged.

SUMMARY OF THE INVENTION

[0006] Unexpectedly, it has now been found that the corrosion rate ofthe salt melt pipes can he significantly lowered if the mixture of ureawith melamine and its decomposition takes place not outside, but insidethe central pipe. Contrary to the original assumption that the flowdirection of the melamine melt is such as indicated in WO 99/00374, ithas been found that the flow direction of the melamine melt in thearrangement according to the invention is exactly the reverse, the meltin fact flows upward within the central pipe and downward outside thecentral pipe.

[0007] The supply of heat necessary for the overall endothermic reactiontakes place by means of the heating pipes arranged outside the centralpipe during the movement of the melt downward, so that in the lower partof the reactor an approximately 3-30° C., preferably 5-15° C., highertemperature prevails than in the upper part. The fact that the melaminemelt in the upper part of the reactor, where it is removed via anoverflow, is colder than in the lower part means a further advantagecompared with the arrangement according to WO 99/00374, since themelamine melt in the subsequent sections has to be cooled less, and theequilibrium position of the melt at the lower temperature is shifted inthe direction of the melamine, so that fewer by-products are formed.

[0008] The invention accordingly relates to a process for the productionof melamine by pyrolysis of urea in a high-pressure reactor having avertical central pipe with formation of a melamine melt, which ischaracterized in that

[0009] the melamine melt circulating in the reactor mixes in the lowerregion of the reactor with a urea melt introduced into the reactor frombelow and optionally introduced NH₃,

[0010] the reaction mixture formed, consisting essentially of melamine,NH₃, CO₂ and optionally reaction intermediates, flows upwards from belowin the central pipe,

[0011] the reaction mixture formed emerges from the central pipe in theupper part of the central pipe,

[0012] the separation between melamine and off-gas takes place at thetop of the reactor above the central pipe,

[0013] a part of the melamine emerging at the top from the central pipeflows downwards in the annular space between the central pipe andreactor wall and the remainder is expelled for further work-up,

[0014] the off-gases are expelled at the top of the reactor.

DESCRIPTION OF THE DRAWINGS

[0015] A more complete appreciation of the invention, and many of theattendant advantages thereof, will be readily apparent as the samebecomes better understood by reference to the following detaileddescription when considered in conjunction with the accompanyingdrawing, wherein:

[0016]FIG. 1 is a side schematic view of a reaction according to theinvention.

[0017]FIG. 2 is a side view of a section of a reactor according to theinvention with a cone-shaped distribution plate and flow guide plates.

[0018]FIG. 3a is a side view of an alternate reactor according to theinvention show introduction of a urea melt.

[0019]FIG. 3b is an end cross-sectional view of the reactor of FIG. 3a.

[0020]FIG. 4 is a cross-sectional view of an alternative reactoraccording to the invention showing introduction of urea via annulargaps.

DETAILED DESCRIPTION

[0021] To carry out the process according to the invention, urea, whichpreferably arrives as ammonia-saturated urea melt from a urea scrubber,having a temperature of approximately 135-250° C. is introduced into themelamine reactor from below. Together with the urea, NH₃ is optionallyintroduced into the reactor from below. The molar ratio of the NH₃optionally fed to the melamine reactor to the urea fed is approximately0-10 mol, preferably approximately 0-5 mol, more preferablyapproximately 0-2 mol of NH₃/mol of urea. The pressure in the melaminereactor, depending on the chosen temperature range, ranges fromapproximately 50-350 bar, preferably from approximately 80-250 bar.

[0022] The temperature in the melamine reactor, depending on the chosenpressure range, ranges from approximately 320-450° C., preferably fromapproximately 320-400° C., more preferably from approximately 330-380°C.

[0023] The melamine reactor is a tank reactor having a vertical centralpipe. The urea melt introduced into the central pipe from below and theoptionally introduced NH₃ preferably flow against a distribution plateinstalled in the lower part of the central pipe and then further eitherpast the distribution plate or through openings or nozzles which arearranged in a retaining device, for example a retaining plate for theattachment of the distribution plate, on the inlet pipe for urea andNH₃, through the distribution plate in the direction of the centralpipe. The reactants mix in the interior of the central pipe with themelamine melt circulating in the reactor and likewise flowing into thecentral pipe from below.

[0024] As a result of the intensive mixing of the cool urea melt withthe hot, circulating melamine melt in the central pipe, warming of thereactants occurs, and the urea pyrolyses over the height of the reactorto give melamine and off-gas, mainly containing NH₃ and CO₂. Since theformation of melamine is endothermic, the amount of the melaminecirculating in the reactor must be so large that, as a result of thelowering of the temperature of the melamine during the mixing of thereactants and during the pyrolysis of the urea, the danger ofsolidification of the melamine does not exist.

[0025] The desired temperature profile in the reactor can be establishedby means of the amount of urea introduced, the temperature of the saltmelt and the direction of circulation of the salt melt in thedouble-jacket pipes.

[0026] In addition, it is possible to attach, at the bottom of thereactor or in the central pipe itself, fittings, distribution plates orflow guide plates or the like, which make possible a comparativemoderation of the flow in the re-routing of the melamine melt from theannular space into the central pipe, a better distribution of the meltflows and the comparative moderation of the bubbles within the centralpipe, and also a better separation between melamine melt and off-gas onemergence from the central pipe and at the top of the reactor.

[0027] In the upper part of the reactor, the separation between off-gasand liquid melamine takes place. The melamine melt can emerge there bothat the upper end of the central pipe and additionally through sideopenings in the central pipe into the annular space between the centralpipe and reactor inner wall.

[0028] A part of the melamine flows downward in this annular space,while the remaining melamine melt is expelled from the reactor via anoverflow for further work-up. The off-gases are continuously removed atthe top of the reactor, preferably in the direction of the ureascrubber. Advantageously, in the region of the separation betweenoff-gas and liquid melamine, baffles or gratings are arranged as acalming zone and for the improvement of the separation action.

[0029] In the annular region between the central pipe and the reactorwall are usually situated vertical heating pipes that aid in providingto the reactor the amount of heat necessary for the endothermicreaction. A part of the melamine melt overflowing from the central pipemoves downwards in the annular space on account of the higher densityand mixes again with introduced urea in the lower central pipe region,which brings about an internal circulation in the reactor.

[0030] The remaining melamine, which is continuously discharged via anoverflow at the top of the reactor, is worked up in any desired mannerand solidified. This can be carried out, for example, by releasing thepressure of the melamine saturated with ammonia at a temperature whichlies barely above its pressure-dependent melting point, bysolidification in a fluidized bed or by quenching with water, withliquid or gaseous ammonia or by sublimation and subsequent desublimationfrom the gas phase.

[0031] A further aspect of the invention is a reactor for the productionof melamine by pyrolysis of urea, comprising a vertical reactor bodyhaving a central pipe, feed lines for urea and optionally NH₃ attachedin the lower part of the reactor, drain lines for the melamine formedand for the off-gases containing NH₃ and CO₂ attached in the upper partof the reactor, heating appliances and measuring and control appliances,in particular for temperature, pressure, flow quantities and heightlevel of the melt, characterized in that one or more outlet openings forthe supply of urea melt and optionally NH₃ are arranged within thecentral pipe.

[0032] In the lower part of the central pipe, a distribution plate forthe distribution of the inflowing urea and of the optionally introducedNH₃ is preferably installed. The distribution plate can either bedesigned as a flat plate, or alternatively, for better distribution ofthe upwardly flowing urea stream and of the likewise upwardly flowingmelamine stream, have any desired geometrical shapes, such as, forexample, the shape of a pyramid, of a half-dish or preferably the shapeof a cone.

[0033] It is particularly advantageous if the gap between thedistribution plate (3) and the outlet opening of the inlet pipe for theurea melt, and optionally NH₃, is as small as possible, for example anannular gap of about 3-13 mm cross-section or openings or nozzles whichare arranged in a retaining device, for example a retaining plate forthe attachment of the distribution plate to the inlet pipe for urea andoptionally NH₃. The openings or nozzles can have any desired geometricalshape and are, for example, circular, annular or in the shape of anannular gap. The openings or nozzles are dimensioned such that theemergence rate at the openings or nozzles is about 0.2-10 m/sec,preferably about 1-5 m/sec, more preferably about 0.5-1 m/sec, so thatthe reactants in the melamine are finely divided. With this arrangement,a higher emergence rate for urea, and optionally NH₃, is achieved, whichmakes possible a better, more intensive and even more homogeneous mixingof the reactants with the outwardly flowing melamine melt. After theemergence from the inlet pipe, the urea stream is preferably diverted inthe direction of the central pipe, such that it flows in the same flowdirection as the melamine. The flowing-in of the melamine meltcirculating in the reactor from the annular space between the reactorwall and central pipe into the mixing zone with the urea melt, and theoptionally introduced NH₃, can be made possible, for example, by sideopenings in the lower region of the central pipe.

[0034] One possible embodiment of the reactor having a flat distributionplate in the central pipe is shown schematically in FIG. 1. FIG. 2 showsa preferred embodiment of the distribution plate in the form of a coneand the incorporation of flow guide plates. FIGS. 3a and 3 b show, inthe upper part, the introduction of the urea melt by means of nozzlesinto the interior of the central pipe and in the lower part across-section, FIG. 4 shows the introduction of urea via annular gaps inthe cross-section.

[0035] In FIGS. 1 to 4, the following reference numerals are used: (1)melamine reactor, (2) central pipe, (3) distribution plate, (4) heatingpipe, (5) annular space, (6) flow guide plates, (7) supplied urea melt,(8) NH₃ gas, (9) off-gases, (10) melamine melt for further work-up, (11)fittings, (12) baffle, and (13) nozzle or annular gap.

[0036] The reactor comprises a corrosion-resistant material or is linedwith corrosion-resistant material, for example titanium.

[0037] It is possible to attach, at the reactor bottom, in the centralpipe and/or in the separating zone at the top of the reactor, fittings,distribution plates, flow guide plates or the like, which make possiblea comparative moderation of the flow in the redirection of the melaminemelt from the annular space into the central pipe, a better mixing ofurea and melamine melt, a comparative moderation of the bubble sizewithin the central pipe and on emergence from the central pipe, and abetter separation between melamine melt and off-gas at the top of thereactor.

[0038] The vertical heating pipes (4), through which the heat necessaryfor the reaction is provided, are preferably double-jacket pipes inwhich a salt melt circulates. Here, the supply of the salt melt can becarried out either via the inner pipe cross-section and the dischargevia the outer pipe jacket or in the reverse flow direction.

[0039] As a result of the mixing and reaction of the reactants withinthe central pipe, their corroding action comes to bear to a much lesserextent. For example, in a melamine reactor according to the presentinvention, such as shown schematically in FIG. 1, the decrease in thepipe wall thickness of those heating pipes (4) which lie next to thecentral pipe (2) is approximately 0.1 mm/year at a capacity of 2.5 t ofmelamine/h. In comparison, the reduction in the pipe wall thickness withthe same high throughputs, but with mixing of the reactants outside thecentral pipe (2), is up to approximately 0.9 mm/year.

[0040] The melamine melt circulating in the reactor serves as aheat-transfer medium for the urea melt introduced into the reactor.Here, an overall decreasing temperature profile is establishedcorresponding to the proceeding urea pyrolysis reaction over the heightof the central pipe, i.e. in the vicinity of the melamine overflow fromthe reactor a lower temperature prevails than at the reactor bottom. Themelamine outlet temperature from the reactor is therefore lower than inmost melamine processes. It preferably ranges from 330 to 380° C., morepreferably from 340 to 370° C. A particular advantage of the reverseflow direction is the low outlet temperature of the melamine melt fromthe synthesis reactor, which can only thus be run at a lower temperaturethan in the previously known processes. The melamine synthesis reactoracts in the upper part as a precondenser. Thus the precondensed melaminemelt separated from the off-gases arrives even from the start with alower by-product content for the next working-up steps.

[0041] Moreover, as a result of the clean liquid flow - as opposed to atwo-phase flow with reversed direction of circulation of the melaminemelt—a lowering of the pressure loss in the annular space between thesalt melt pipes, and thus an increase in the amount of circulation inthe reactor, is achieved. The heat transfer from the salt melt to themelamine melt is thereby improved.

[0042] In addition, the possibility exists with fittings in the centralpipe of influencing the flow and the bubble size and distribution of theoutwardly flowing reaction mixture by means of which a furtherimprovement in the substance and heat transfer can be achieved.

1. Process for the production of melamine by pyrolysis of urea in ahigh-pressure reactor having a vertical central pipe with formation of amelamine melt, characterized in that the melamine melt circulating inthe reactor mixes in the lower region of the reactor with a urea meltintroduced into the reactor from below and optionally introduced NH₃,the reaction mixture formed, consisting essentially of melamine, NH₃,CO₂ and optionally reaction intermediates, flows upwards from below inthe central pipe, the reaction mixture formed in the upper part of thecentral pipe emerges from the central pipe, the separation betweenmelamine and off-gas takes place at the top of the reactor above thecentral pipe, one part of the melamine emerging above from the centralpipe flows downwards in the annular space between the central pipe andreactor wall and the remaining part is expelled for further work-up, theoff-gases are expelled at the top of the reactor.
 2. Process accordingto claim 1, characterized in that the urea melt and optionally the NH₃are introduced into the central pipe from below.
 3. Process according toclaim 1, characterized in that the temperature of the melamine melt atthe upper end of the central pipe is lower than at the lower end. 4.Process according to claim 1, characterized in that the urea melt andoptionally NH₃ introduced into the central pipe flow against adistribution plate in the lower part of the central pipe.
 5. Processaccording to claim 1, characterized in that the melamine meltcirculating in the reactor enters into the central pipe through sideopenings arranged in the lower part of the central pipe, and flowsupwards in the central pipe and downwards in the annular space betweenthe central pipe and reactor wall.
 6. Reactor for the production ofmelamine by pyrolysis of urea, consisting of a vertical reactor bodyhaving a central pipe, feed lines for urea melt and optionally NH₃installed in the lower part of the reactor, drain lines for the melaminemelt formed and for the off-gases consisting essentially of NH₃ and CO₂in the upper part of the reactor, heating appliances and optionallymeasuring and control appliances, in particular for temperature,pressure, flow quantities and height level of the melamine melt,characterized in that one or more outlet openings for the supply of ureamelt and optionally NH₃ are arranged in the lower part and within thecentral pipe.
 7. Reactor according to claim 6, characterized in that adistribution plate is installed in the lower part of the central pipe,above the supply pipes for urea and optionally NH₃.
 8. Reactor accordingto claim 7, characterized in that the distribution plate is providedwith one or more openings in the direction of the central pipe for thepassage of the urea melt and optionally NH₃.
 9. Reactor according toclaim 7, characterized in that the distribution plate has the form of aflat plate, a cone, a pyramid or a half-dish.
 10. Reactor according toclaim 6, characterized in that fittings for the comparative moderationof the flow and the comminution of the gas bubbles and for theimprovement of the mixing are situated in the central pipe.
 11. Reactoraccording to claim 6, characterized in that a baffle and above it acalming zone are situated at the top of the reactor above the centralpipe in the separation zone.
 12. Reactor according to claim 6,characterized in that the central pipe has side openings in its lowerand/or upper part.